The present invention relates to a novel welded structure for use at very low temperatures and, more particularly, to a welded structure such as container produced by welding from austenitic stainless steel for housing a superconducting magnet.
Hitherto, austenitic stainless steel has been used as a material for use at a very low temperature of 4.2.degree. K. Welding rods or wires of the groups of D308, 308L, 316, 316L as specified in JIS (Japanese Industrial Standard) have been used as the welding material for welding this material. In general, in order to avoid hot weld crack of the deposited metal obtained after the welding, the compositions of these welding materials are so adjusted as to cause crystallization of at least several percent of delta ferrite.
The container for superconducting magnet is strictly required to be non-magnetic. Therefore, when the aforementioned material for use at very low temperatures is used as the material of the superconducting magnet container, the crystallization of delta ferrite, which is ferromagnetic, is not preferred. The present inventors have found also that the delta ferrite is one of the factors which impairs the toughness of the deposited metal at very low temperatures. Therefore, the crystallization of delta ferrite is not preferred also from the view point of toughness of the deposited metal. By the above-mentioned reasons, the deposited metal in the prior art was not satisfactory for use at very low temperatures.